1. Field of the Invention
The present invention relates generally to the testing of oil and gas wells to determine the natural formation pressure of the subsurface formation and the producing characteristics of the subsurface formation, and more particularly, but not by way of limitation, to such techniques which are especially applicable to early evaluation testing of an open borehole soon after the borehole is drilled into the subsurface formation of interest.
2. Description Of The Prior Art
During the drilling and completion of oil and gas wells, it is often necessary to test or evaluate the production capabilities of the well. This is typically done by isolating a subsurface formation which is to be tested and subsequently flowing a sample of well fluid either into a sample chamber or up through a tubing string to the surface. Various data such as pressure and temperature of the produced well fluids may be monitored downhole to evaluate the long-term production characteristics of the formation.
One very commonly used well testing procedure is to first cement a casing in the borehole and then to perforate the casing adjacent zones of interest. Subsequently the well is flow tested through the perforations. Such flow tests are commonly performed with a drill stem test string which is a string of tubing located within the casing. The drill stem test string carries packers, tester valves, circulating valves and the like to control the flow of fluids through the drill stem test string.
Typical tests conducted with a drill stem test string are known as draw-down and build-up tests. For the "draw-down" portion of the test, the tester valve is opened and the well is allowed to flow up through the drill string until the formation pressure is drawn down to a minimum level. For the "build-up" portion of the test, the tester valve is closed and the formation pressure is allowed to build up below the tester valve to a maximum pressure. Such draw-down and build-up tests may take many days to complete.
There is a need for quick, reliable testing procedures which can be conducted at an early stage in the drilling of the well, preferably before casing has been set. This is desirable for a number of reasons. First, if the well is proven not to be a commercially successful well, then the cost of casing the well can be avoided or minimized. Second, it is known that damage begins occurring to the subsurface formation as soon as it is intersected by the drilled borehole, and thus it is desirable to conduct testing at as early a stage as possible.
On the other hand, there are a number of difficulties encountered in the testing of open, uncased boreholes. This is particularly true for subsea wells. Due to safety considerations it is often considered undesirable to flow test an open hole subsea well through a drill stem test string.
Also, it is not convenient to do conventional draw-down, build-up testing in an open hole situation because the pipe is full of drilling mud which would have to be circulated out. It is preferable to conduct a test with a safe dead well which is completely kept under control due to the presence of the column of heavy drilling mud.
Also, at this early stage of drilling the well, there is a need for a test which can be conducted very rapidly so that repeated tests can be conducted as the well is drilled to quickly evaluate the various subsurface formations which may be intersected as the well is drilled. Conventional draw-down and build-up tests can take several days to complete, and they substantially interrupt the drilling process.